Ethylbenzene (EB), para-xylene (PX), ortho-xylene (OX) and meta-xylene (MX) are present together in many C8 aromatic product streams from chemical plants and oil refineries. While all these species have important uses, market demand for paraxylene, used extensively as starting material for making synthetic fibers, is greater than for the other C8 aromatic isomers.
Given the higher demand for PX as compared with its other isomers, there is significant commercial interest in maximizing PX production from any given source of C8 aromatic materials. However, there are two major technical challenges in achieving this goal of maximizing PX yield. First, the four C8 aromatic compounds, particularly the three xylene isomers, are usually present in concentrations dictated by the thermodynamics of production of the C8 aromatic stream in a particular plant or refinery. As a result, the PX production is typically limited to the amount originally present in the C8 aromatic stream, which is, again in the typical case, approximately 24 mol % at thermal equilibrium, unless additional processing steps are used to increase the amount of PX and/or to improve the PX recovery efficiency. (Methods of making paraxylene with higher selectivity, such as by toluene alkylation with methanol, are well-known.) Secondly, the C8 aromatics are difficult to separate due to their similar chemical structures and physical properties and identical molecular weights.
A variety of methods are known to increase the concentration of PX in a C8 aromatics stream. These methods normally involve a loop system comprising a separation step, in which at least part of the PX is recovered (and removed from the system in a PX-enriched stream), leaving a PX-depleted stream, the latter being sent to a xylene isomerization step, in which the PX content of the PX-depleted stream is returned back towards thermal equilibrium concentration and recycled to the separation step.
The separation step is typically accomplished using fractional crystallization techniques, which is based on the difference on the freezing points of the C8 aromatic isomers, or adsorption separation techniques, which is based on the selectivity of adsorbant for one isomer over another. Amongst the well-known adsorption separation techniques are the UOP Parex™ Process and the IFP Eluxyl™ Process.
A prior art system including the separation step and isomerization steps referred to above generally will include the use of numerous fractionation towers, e.g., a reformate splitter, a benzene recovery tower, a toluene recovery tower, a xylene rerun tower, an isomerization unit heptanizer, and one or more towers associated with the adsorption separation unit, e.g., Parex™ adsorptive separation unit(s). A system comprising a Parex™ adsorptive separation unit using PDEB (para-diethylbenzene) as a desorbent (“heavy” Parex™ adsorptive separation unit) will have an extract tower, raffinate tower(s) and finishing tower(s) while a system comprising a Parex™ adsorptive separation unit using toluene as a desorbent (“light” Parex™ adsorptive separation unit) only needs the extract and raffinate towers, since the extract tower separates out both the toluene in the desorbent stream as well as trace toluene in the xylene feed. In a plant using both types of units the light extract tower can serve as the finishing tower for the heavy unit.
The isomerization step typically is accomplished by contact with a molecular sieve catalyst, such as ZSM-5, under appropriate conditions to convert a para-xylene-depleted mixture of C8 aromatic hydrocarbons to thermodynamic equilibrium amounts. Historically xylene isomerization has been accomplished in the vapor phase, however recently liquid isomerization units have found increasing use in para-xylene separation systems.
It is known that liquid phase isomerization technology can reduce energy usage in an aromatics plant by reducing the amount of feed to vapor phase isomerization. Vapor phase isomerization requires more energy due to the phase change in the isomerization process. In addition, vapor phase isomerization requires more fractionation energy in the isomerization system's heptanizer and xylene rerun tower.
It is known in the prior art to integrate a liquid isomerization unit with a vapor phase isomerization unit by taking a slip stream from the para-depleted mixed xylenes product of a raffinate tower downstream of a paraxylene recovery unit (whether it is Eluxyl™ unit, Parex™ adsorptive separation unit, or light and heavy Parex™ adsorptive separation unit in parallel), and then passing the para-depleted mixed xylenes through the liquid phase isomerization unit to provide an equilibrium mixture of xylenes, which is then recycled back to the paraxylene recovery unit(s). Thus, the liquid phase isomerization unit is used essentially to supplement the vapor phase isomerization unit, but otherwise the processing in the xylenes loop is the same.
The present inventor has realized that the raffinate from the paraxylene recovery unit may be sent directly to the liquid isomerization unit without one or more intervening fractionation towers. Thus, in the integration of liquid phase isomerization and adsorptive separation there does not need to be a raffinate tower as used in conventional paraxylene recovery systems.